Mozilla VPN Flatpack 2.30.0 fail to update DNS (DNS leak) Fedora 42

[thematthew1]

Hey folks,

I toyed with this a bit but couldn't get to the bottom of the issue.

Steps to repeat:
use mozilla vpn flatpack with any server location on fedora 42 (uncertain if it's a fedora issue specifically at this time)
use a dns leak tester (I used mullvads)
dns reports non vpn dns

I confirmed no issues on android. Additionally no issues with mullvads client while not installed with flatpack.

I believe the issue may lie in the sandbox / flatpack permissions not being sufficient to overwrite resolv files.

Thanks

[oskirby]

Hey! Thanks for the bug report.

Would you be able to share logs with us that might help diagnose the issue? You should be able to access the logs by clicking on the Settings icon, navigating to the Help menu and clicking View Logs

For what it's worth, modern Linux systems don't really work by overwriting the /etc/resolv.conf file anymore. Instead they rely on the systemd resolver, which is typically reachable at 127.0.0.53. The configuration of this resolver is typically handled over D-Bus or indirectly through other services like the Network Manager (as is the case for the Flatpak builds).

It would also be helpful if you could let us know which browser you are using for the leak check, as some browsers have been known to bypass the system resolver configuration and do their own thing.

[thematthew1]

Thanks for the quick reply.

I'm using Firefox 142.0.1 provided by standard fedora repos. Enable DNS over HTTPS using: is set to off.

I saw as much regarding D-bus via network manager. Flatpack persmissions seem to be set correctly to talk with org.freedesktop.NetworkManager. I tried giving it ownership, to no avail. Mullvad provides an rpm which works flawlessly (no leaks). My instinct was that perhaps the Flatpack sandbox did not contain the correct tools to talk systemd resolver, but I have to admit a high level of ignorance to how all that works!

I checked the logs I perhaps found the smoking gun:
"[18.09.2025 16:53:25.456] (NetworkManagerController) Error: peer configuration failed: ipv4.dns: 1. DNS server address is invalid"

attached complete log with this entry.

mozillavpn-2025-9-18_1.log

[oskirby]

That certainly does seem like the smoking gun here, and based on the logs it looks like you have configured a custom DNS address, would you be willing to share the value you have configured as your DNS server?

[thematthew1]

Sure thing. Looks like the flatpack is not updating the stystemd-resolve:

The DNS is set in the mozilla client to "use standard DNS"

I've not modified anything system level to prevent updating.

With FirefoxVPN client:

 systemd-resolve --status

Global
         Protocols: LLMNR=resolve -mDNS -DNSOverTLS DNSSEC=no/unsupported
         resolv.conf mode: foreign
         Current DNS Server: 192.168.50.1
         DNS Servers: 192.168.50.1

Using the Mullvad Client (fedora binary):

      Protocols: LLMNR=resolve -mDNS -DNSOverTLS DNSSEC=no/unsupported
      resolv.conf mode: foreign
      Current DNS Server: 100.64.0.23
      DNS Servers: 100.64.0.23

note: this dns is publicly listen on mullvads website.

When attempting to override DNS in mozilla client, I am unable to connect to any server and get a timeout of sorts.

Let me know if there's anything more specific you'd like me to provide